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37 1. Force induced by natural or mechanical effects. It is ize the limits of the existence of backlayering. These experi- characterized by the air velocity obtained upstream of ments have been associated with a CFD technique. The good the fire, U. correlation obtained shows that the control of the boundary 2. Buoyancy forces developed in the fire plume, which conditions in the experiments was correct and that they could are induced by the gases' expansion resulting from the be correctly described in order to perform numerical simula- high temperature. The fundamental characteristic is tions. It is to be noted that the characterization of these bound- given by the density difference between the air and the ary conditions for full-scale tests remains a problem. hot gases, . Using a small-scale model to design a tunnel ventilation To represent the turbulent longitudinal flow, it appears system may be limited for two primary reasons: necessary to use the Reynolds number Re: Technical conclusions are relative to the similarity law(s) Re = UDh v (14) used. A fire is a complex phenomenon and its represen- tation cannot be limited to one or two global relations. where: HRR representation remains an unsolved problem. Dh represents the hydraulic diameter, and It is not correct to conclude that the lack of total similarity leads v represents the fluid cinematic viscosity. to unrealistic results. For example, the conclusions drawn from small-scale experiments performed in the Channel Tunnel on The effect of buoyancy forces are partially represented by shuttles have been confirmed later through full-scale tests. the Froude number, Fr: Fr = U 2 gDh (15) The representation of realistic situations with reduced- scale models depends on the number of similarity laws taken where g represents the gravity acceleration. into account. As only one parameter is simulated (Froude or Richardson number), the global validity of this kind of study The Froude number modified with the density differences is not accurate. The application of this technique to full-scale represents the gravity effects on fluid motions, resulting in situations is not immediate. As an example, the conclusions the Richardson number: drawn from the study concerned with trap doors or single- point extraction openings, recently done in France, have been Ri = ( gDh U 2 ) ( ) (16) applied to other projects because they provide valuable answers concerning the relative capacities of the various systems; Other parameters may be used to study phenomena on however, absolute results were not used. reduced-scale models. For example, the Grashof number is a combination of the Reynolds and the Richardson numbers: The second case is the use of small-scale models for re- search. The conclusions of such studies are generally limited to Gr = ( gDh 3 v 2 ) ( ) (17) the model studied. The transposition of the established laws to full-scale situations needs reference experiments. Therefore, The Reynolds condition is generally limited to checking that the interest of these models is to show that general laws can be the Reynolds numbers in the model are sufficient to ensure the drawn from the study of specific situations, which also give turbulent character of the longitudinal airflow. analytic form for these laws (e.g., existence of backlayering versus source characteristics and longitudinal air velocity.) The thermal exchanges with the walls are difficult to model exactly as they would appear in an actual tunnel. In general, the validity of a study based on the use of mod- els is directly linked to the interpretation of the similarity law. The relation between the backlayering distance, the local slope, the heat release, and the thermal exchanges with the LARGE-SCALE EXPERIMENTAL FACILITIES walls has been demonstrated using small-scale models. Den- sity change represents temperature and vertical velocity as Such tests can be considered to be somewhere in between a the function of burned gases. full-scale road tunnel test and small-scale laboratory tests. An example of such a facility is a laboratory tunnel of Carleton The fire source can be modeled by a flux mixing a light gas University, located in Almonte, Ontario, Canada, which is used (generally helium) and air or nitrogen. These models cannot for performing large-scale experiments. The tunnel is 37.5 m represent thermal exchanges with the walls. The isothermal (123 ft) long and the cross section is 10 m (32.8 ft) wide and source does not take into account the physics of fires. In real- 5.5 m (18 ft) high. The tunnel has a shutter opening [3.8 m wide istic situations, the combustion temperature is related to the (12.5 ft) and 4.0 m (13.1 ft) high] and two louvered openings vertical velocity. In the experiments, these two parameters are [1.2 m wide (3.9 ft) and 4.5 m (14.8 ft) high] at the east end. not dependent. Such experiments have been used to character- Figure 12 is a schematic diagram of the tunnel facility.